Progress in reconstructing climate in recent millennia

That’s not the start of a joke, but it is a good jumping off point for a discussion of the latest publication on paleo-reconstructions of the last couple of millennia. As has been relatively widelyreported, Mike Mann and colleagues (including Ray Bradley and Malcolm Hughes) have a new paper out in PNAS with an update of their previous work. And this is where the question posed above comes in: the difference is that with time scientists can actually make progress on problems, they don’t just get stuck in an endless back and forth of the same talking points.

We discussed what would be required in an update of these millennial reconstructions a few months back and the main principles remain true now. You need proxies that are a) well-dated, b) have some fidelity to a climate variable of interest, c) have been calibrated to those variable(s), d) that are then composited together somehow, and e) that the composite has been validated against the instrumental record.

The number of well-dated proxies used in the latest paper is significantly greater than what was available a decade ago: 1209 back to 1800; 460 back to 1600; 59 back to 1000 AD; 36 back to 500 AD and 19 back to 1 BC (all data and code is available here). This is compared with 400 or so in MBH99, of which only 14 went back to 1000 AD. The increase in data availability is a pretty remarkable testament to the increased attention that the paleo-community has started to pay to the recent past – in part, no doubt, because of the higher profile this kind of reconstruction has achieved. The individual data-gatherers involved should be applauded by all.

The increase in proxy records allows a whole bunch of new things to be done. First off, the importance of tree rings can be tested more robustly. With the original MBH98 proxies, there was only enough other data to go back to 1760 if you left out the tree rings. The match was pretty good over multi-decadal periods, but the interannual variability was much larger without tree-rings. Now though, the Northern hemisphere land temperature reconstructions without tree rings can go back to 1500 AD or 1000 AD depending on which of two methodologies are used. For the NH land and ocean target, it’s even possible to get a coherent non-tree ring reconstruction back to 700 AD! As before, there are some differences (notably in the 17th Century where the tree rings indicate colder temperatures), but the recent warming is anomalous regardless.

Secondly, you can screen records and pick targets more finely: do you want only records that match local temperatures? Done. You want to get a handle on global and southern hemisphere means as well as the northern hemisphere? Done. Other screens could easily be implemented.

The two methodologies used themselves span the range of different approaches that people have used. ‘Composite and scale’ (CPS) is perhaps the simplest method – it is basically an average of all the temperature proxies scaled to the target time series. The other method is denoted ‘Error in variables’ (EIV) in this paper, but is really a simplified application of the RegEM climate field reconstruction method used in a couple of more recent papers. It is essentially a fancy multiple regression to the target time series that can incorporate non-local proxies as well. The point of using two methods is to demonstrate what is, and what is not, robust, and to give an idea of what the structural uncertainty in these estimates is – something not easily calculated using standard statistics. That uncertainty is clearly larger as you go back in time, and larger still for the southern hemisphere.

Other improvements over previous work are that more proxy data sets go past 1980, and so calibration up to 1995 is possible. That allows more of the recent trends to feed into the calibration and highlights the so-called divergence problem in some (but not all) recent tree-ring records. That divergence is significantly lessened without tree-rings or using the EIV method.

Figure: Spaghetti plot of the new reconstructions over a) 1800 and b) 1000 years
along with selected older ones for comparison.

So what does it all mean? First off, this paper (like MBH98 before it) is not an attribution study. That means that the reasons for any of the ups-and-downs in the records are not demonstrated by these papers alone. Attribution of the recent trends (as discussed in IPCC AR4) to anthropogenic effects has mostly focussed on the last 150 years and did not use any paleo-data. Nonetheless, there have been a couple of key studies that have used this kind of data along with simple energy balance models (Crowley, 2000; Hegerl et al, 2006 for instance) and it will be interesting to see if this new reconstruction will make any difference to their conclusions.

Secondly, in comparison with previous reconstructions, the current analysis does not provide many surprises. Medieval times are warmer than the Little Ice Age as before, and a little warmer using the EIV method than was the case in MBH99. The differences in the 11th Century are on the order of a couple of tenths of a degree – well within the published error bars in IPCC TAR though. Interestingly, there are quite rapid and strong drops in temperature near 1100 AD and around 1350 AD which may make interesting case studies for attribution to solar or volcanic forcings in future. Overall, there are a few more wiggles than before, but basically nothing much has changed. (Though one should always be aware of the maxim that one person’s noise is another person’s signal).

Finally, while the headline numbers ‘likely warmest since XXXX’ are of some contextual value, they aren’t the real point of this kind of study. Most of the interesting work – looking for patterns associated with solar forcing say – will start when the spatial patterns of temperature change start to be discerned – and that is still a work in progress.

So, onto the inevitable discussion! One test of whether that discussion is more political than scientific will be the extent to which people acknowledge the progress that has been made. Repetitions of tired and oft-debunked one-liners will be telling!

119 Responses to “Progress in reconstructing climate in recent millennia”

Gavin, what is your opinion on the Finnish Lake sediment proxies? They certainly jump out when looking at the series used. Would you consider them quality temperature proxies?

[Response: I have no particular opinion, but leaving them out (and a few other potentially problematic ones) was tested as described in the SI (p2 and fig S8). It makes a little difference, but nothing particularly striking. It’s worth pointing out that in these kinds of projects there are no guarantees – all you can do is attempt to find out how robust answers are to various issues (this is one, but so is the dependence on tree ring data, reconstruction methodology, etc, etc.) and seeing what decisions make a significant difference to the final answer. If they do, then obviously you are constrained about what can be concluded, while if they don’t matter, it’s not worth delving deeper. – gavin]

In the graphs presented here, the proxies appear not to reproduce the 20th century temperature rise. Why is this? If the 20th century temperature rise is not reproduced by the proxies in the 20th century, why would we expect them to reproduce a temperature rise in the past similar to today’s, if such a temperature rise did occur?

[Response: Please read the actual paper and look at the figures (particularly fig 2) and you will see a close up of what the 20th century calibration/validation looks like. There is a well known problem with tree-ring divergence (in the CPS method), but that is much less of a problem if the tree-rings aren’t used, and even less if one uses the EIV method. Thus if your criteria for acceptance is a good late 20th C trend, then I’d suggest focusing on the EIV method. However, in the absence of a good explanation of the modern divergence issue (which is not universal), it seems prudent to keep track of the differences in the two methodologies as a heuristic estimate of the structural uncertainty. – gavin]

When the study was published I discussed with Mia Tiljander and she stated that human impact, especially land cover change, agriculture, accelerates erosion, more material flows to lake sediments, and thereafter the lakes are of no use in proxy reconstructions anymore, Korttajärvi from 1720 on, most of the other 180.000 Finnish lakes from 1850s on.

[Response: That is, I presume, why Mann et al did a test that didn’t include them. – gavin]

I read this article 2 days ago at my local library. Although I scanned it quickly, my initial conclusion was that Broecker thinks Ruddiman’s hypothesis (early agriculture responsible for much climate change) is weak, to put it politely.

JM, I know nothing about viticulture but I assume, perhaps wrongly, that the grapes grown in the UK in the MWP were somewhat different to those grown now, today’s having been selected over time for resistance to cold/diseases etc. So the fact that vineyards are now able to be located in more northerly locations in Britain than was apparently the case in the MWP doesn’t necessarily relate to temperature at all.

Actually at Loch Maree in Northern Scotland they grow semi-tropical plants outside at Inverewe Gardens. All due to the gulf-stream, good cover and mild Winters apparently. So you can get pockets of temperate climate in unpromising places without it having to mean anything at all.

[edit] Evidently most of the discussion is in comparison between present proxies and present instrumental measured temperatures. The other is in the contradictions that sometimes happens between proxies. If you have 2 and minus 2, the statistical medium is 0. But reality is 2 and minus 2.

Please take a look at the longer discussion at Hot Topics that I mentioned in #42, with the various caveats around the interpretations of the data. However, it really helps to have the actual book, with the detailed archaeological maps, to assess whether or not this is meaningful, or not. I’m looking forward to Selley’s 2nd Edition, as he says it has more temperature data.

Regarding cold-climate grapes:
1) I’m no oenologist or viticulture expert.
I do live in Northern California, vineyards are in the hills near our house, Napa/Sonomoa are an easy day trip, and we spend a few weeks a year in the Okanagan, the wine area of British Columbia.

See that he shows that different kinds of grapes grow different places, and the zones move, at least in part by temperature. As far as I know (but if an expert knows something, please post), one might breed more cold-resistant grapes, but generally, grape *types* can be very geography/climate/soil/temperature limited. I glaze over when the vintners around here wax enthusiastic about the relatively growability of this grape type versus that type in areas 5 miles apart. {We have a lot of microclimates around here.]

3) The area around Lake Okanagan in B.C. has become a pretty decent wine area, especially in the last few decades. We’re up there 2-3 weeks/year and always try the local wines. Even the WSJ wrote about the vintners there betting on milder climate (correctly).

4) Anyway, I make no claim this is an authoritative proxy, but it is one where there is historical and archeaological information that actually goes back ~2000 years, in one area. The ebb and flow of wineries is an interesting indicator, because people always seem to try to grow grapes as Pole-ward as they can, even if the results are … not so good :-)

The ebb and flow of wineries is an interesting indicator, because people always seem to try to grow grapes as Pole-ward as they can, even if the results are … not so good…

There’s an old adage in the wine business: the best wines are made from grapes grown on warm slopes in cool climates. Winemakers in the newer growing regions have been searching for those sorts of sites. In France and Germany they found them a thousand years ago (or more).

[Response: Surely? Why? Maybe the plague spread faster in cooler conditions? There have been some suggestions that reforestation after the outbreak could have had a climate effect, but it’s all very speculative. – gavin]

re: #55 Bob
Thanks.
Yes, I know Broecker’s earlier objections. I think Bill thinks he’s addressed those via the latest papers, but I don’t know if Broecker has come back with further comments in reply. I stopped in Stanford Bookstore today, but it still only had the previous issue of NS, so I’ll have to wait, unless someone can say how up to date the article is. I.e., does it use Ruddiman’s latest, and Broecker’s comments to that.

Again, as noted, all this stuff is speculation/hypothesis, not strong theory. I really do recommend that people go read the various papers, because it illustrates *real* science as practiced by *real* scientists, where in fact serious experts take different viewpoints. Watching such in action [as opposed to after the dust has settled, however] is extremely educational, especially since:

a) This one is actually reasonably accessible to the non-expert.
b) One need read only a relatively small number of papers to keep up.
c) It helps people learn the difference between real scientific arguments and fake ones.

It makes no sense to argue the issue without reading the papers and the earlier commentary here.

re: #66 grapes gareth
I can believe that [warm slopes, cool climates, which also fits Selley’s North shores of lakes].

My comment was perhaps unfair, but it derives from having spent my early years in PA/NJ, and for many years, thought that “wine” meant pink Catawba grown further North in NY :-).

So, have you read the book, or any of the articles? Bill may or may not be right, but he has done some fairly detailed analyses of the number of hectares/person required at various levels of technology, plausible deforestration/reforestration rates, methane generation from rice paddies, and they don’t seem contradictory with other things I’ve seen, like Brian Fagan’s books.

Reforestration rates are certainly compatible with my personal experience. Our family farm (PA) had a pasture cut from forest in the 1840s, and within 20 years of stopping farming,the forest had reclaimed it to the point you’d never think it had been anything else.

“… The population figure for the entire hemisphere [before contact from Europe] is thought to fall between 48 and 53 million…. no one disputes the sharp downward trajectory in population density …. Estimates of population decline following contact with the Europeans range from 70 to 90 percent and what happened has been called a ‘demographic catastrophe.’”

Some people dispute the high figures, especially for North America, on the basis that hunter-gatherer economics can only sustain a very low population density. There certainly was a bad die-off from European contact, primarily due to the spread of smallpox, as I remember, but I think the 90% figure is controversial. Remember that that the Black Death only killed off 25% of Europe.

Thank you, Dhogaza, I am of course aware that they weren’t all hunter-gatherers. I don’t think you can get up to several tens of millions with family-plot farming, either, especially since we know that most of the territory was in fact forest.

Barton, you seem to be thinking only about North America. Don’t forget about the large agricultural civilizations of Central and northern South America. Even in the North America there were the mound builders of the south/southeast.

Google:
patuxet plague (of ~2000 natives, 1, Squanto, survived, because he wasn’t there. Almost 100% die-off.)
squanto (who taught Pilgrims how to plant corn)
mound builders (Hopewell was earlier, but it’s worth looking at)
Aztec
Tenochtitlan (~200,000 people in 13.5 km^2, in 1519, one of the largest cities in world, 5X bigger than London. “hunter-gatherers” :-); people guess 1M people in the Mexico Basin.
Inca empire

====
A few references:

Jared Diamond, Guns, Germs and Steel … is a must.

Tim Flannery, The Eternal Frontier- An Ecological History of North America and Its Peoples, 2001. Chapters 18-19. Also:

p.289: “And the settlers were to become like Indians in other ways: nearly every colony established itself on the site of an old Indian village. They grew Indian crops such as corn and squash, and according to European observers they even started to farm like Indians.”

p. 303: “In 1634 a violent epidemic of smallpox broke out among the Pequots living inland along the Connecticut River. ‘it pleased God,’ William Bradford wrote, ‘to visit these Indians with a great sickness and such a mortality that of a thousand, above nine and a half hundred of them died.” [Sounds like 95% die-off.]

[Not too far North of you is a town called Bradford Woods; our family farm was next to that. I still have arrowheads. History of Pittsburgh is worth reading; especially track down the lifestyles of Adena and Hopewell cultures which preceded Europeans by 1000+ years. I.e., of course they hunted & fished (they lived in PA, after all!) and farmed.

p.304 “The Mandan tribe, which lived along the upper Missouri River, was blighted by smallpox in the fall of 1837…. cost the lives of 1569 out of 1600 Mandan.” [That’s ~98%. of course, people argue abotu whether this was accidental or on purpose.]

p.305 “But which-the guns, the germs, or the steel-was the most important?”

The answer, Diamond has no doubt, is germs. Although estimates vary widely, by 1492 the human population of the Americas may have reached 57 million, of whom 21 million lived in Mesoamerica. Eighty years later it had shrunk to 18 million….indeed, it is thought that disease had wiped out 90 percent of the people living in Mesoamerica, Peru, and the Caribbean by 1568.” [57->18 ==> ~70% die-off in 80 years, if that’s accurate.]

====
The likely truth, when Europeans arrived:
a) *Most* native Americans lived in societies that farmed at least some of the time. Given corn (teosinte)origin in Mexico & south, if they were growing corn around Boston (and they were), it’s hard to believe there weren’t extensive agricultural communities in the much-easier farming areas all over the Mississippi river basin.

b) Given that native Americans didn’t have cows, sheep, goats, etc [see Diamond], many still hunted/fished, but people living in large villages who raise crops, but also hunt/fish are a far cry from “hunter-gatherers”. [And they were: look up “mound builders”.]

c) California had a bunch of tribes who gathered acorns in preference to raising crops.

d) Unlike Europeans, native Americans didn’t keep the domestic animals that act as disease reservoirs, and had almost no immunity to the diseases when they came.

===
I’m no expert on the population estimates, and the experts do argue, but many of the more current estimates are much higher than they were, say 20 years ago, because the older estimates simply didn’t factor in the decimation that went on. By the time Europeans got to some areas, the natives were dead and forest had regrown. Of course, it’s way more comfortable to think that Europeans settled an empty land sparsely populated by a few hunter-gatherers…

Given the difficulty of estimates, none of this *proves* there were 50M+ pre-Columbian natives.

But for sure, the evidence is overpowering that many natives did agriculture (whether by planting crops or gathering acorns), and hunting/fishing, based in well-established settlements.

Furthermore, in many parts of at least North AMerica, they routinely burned the forests even if they didn’t practice planting; the Californians did so to preferentially promote the nut bearing trees. “The Ancient Forest” expllains this for the lower 48 in detail.

Barton, you really need to read this, horrifying as it is to know. Look at the link I gave you; that gives the range of estimates, and I didn’t pick an extreme.

The “Eden” the whites described was rich because it was farmed, or morelike gardened. Not using horses, not with plowed straight lines, not with Medieval land use patterns, not recognized as such by those who marched through it and took it, telling themselves it was untouched wilderness, just happened to be incredibly rich with things people eat and use — and yet almost empty of people. They’d already died. The diseases moved ahead of the Europeans.

There was plenty of human agricultural activity, and it ceased when the diseases crossed the Americas.

The Willamette Valley in Oregon was routinely burned, too. That’s why it was mostly cleared by the time the first white settlers arrived to farm. In fact, that’s one reason why it looked like such fantastic farm land to them (and, they were right).

“By 1500 an estimated 55 million people lived in the Americas, mostly in naturally forested areas [Denevan, 1992]. The per capita amount of land in cultivation or in fallow was 0.4–1.4 ha/person [Hurt, 1987; Krech, 1999], equivalent to the estimate by Gregg [1988] for Neolithic Europeans. Assuming an average footprint of 0.9 ha, the 55 million people would have cleared 0.5 million km2 and emitted 6.5 Gt C (at an average carbon density of 13 Gt/km2). [153] This estimate, however, omits an important consideration: fire. In contrast to Neolithic Europeans most indigenous Americans kept no livestock (except for Andean llamas and alpacas). Instead, to supplement their needs for nutrition, they repeatedly burned vegetation to maintain grassland, attract game, and promote growth of berries and other foods. The additional ‘‘clearance’’ by burning, generally thought by paleoecologists to have been extensive, would have added considerably to the footprint based only on land under cultivation. [154] On the basis of the added clearance by burning the human forest footprint in the Americas is (arbitrarily) assumed to have been 2 ha, compared to 3 ha in Neolithic Europe. For a population of 55 million the total amount of deforestation just before European arrival would have been 1.1 million km2. Faust et al. [2006] arrived at a nearly identical value. At a mean carbon density of 13 Gt C per million km2 [Houghton, 1999], total deforestation by 1500 would have produced 14 Gt C.”

That compares to an estimate of ~8.2Gt of carbon sequestration for Black Death (1350-1450) and ~7.7 Gt from Late Roman pandemics (200-600).

Ruddiman has pages of analysis talking about China issues, whether specific areas would have gotten refarmed quickly (some did, some didn’t), etc, ending that part with:

“In summary, pandemic-driven reductions in atmospheric CO2 can explain half or more of the 7-ppm drop between 1200 and 1700. Depending on the highly uncertain size of the global mean cooling during this interval, this anthropogenic forcing could account for anywhere between 16% and 66% of the total cooling. In view of the uncertainties in pandemic mortality and in the size of human forest footprints, however, the simulations in Figure 21a should be viewed as a demonstration of the first-order plausibility of the pandemic CO2 hypothesis rather than as a detailed simulation of actual changes.”

Like he says, a demo of first-order plausibility… Unlike statements we hear like “Effect X is caused by Y”, we get “It looks like 16-66% of effect X could be caused by Y”, with large amounts of careful detail to create plausible estimates and error bars.

Main message: if you’re interested in this topic, go read that 37-page paper. I’m no expert to know if it’s right, but it certainly looks like careful work, real science in progress of chewing on hypotheses.

I’m astonished by the attitude that if I just studied the subject, I’d change my mind, and that I don’t know anything about the Central and South American empires or the smallpox epidemics. For the record, I’m not disputing

A) that there were lots of people in the New World
B) that the Europeans introduced smallpox, or
C) that the diseases resulted in a big human die-off.

I’m simply and solely disagreeing with the upper-end numbers quoted.

I’m waiting for someone to quote Stannard’s figure of 100 million dead, which is probably about four times the actual population of the New World back then.

I’d also like someone to give me an example of a continent-wide plague other than the American one which killed off 90% of the population. The Black Death in Justinian’s time? The Black Death in the 14th century? Why is it that this kind of die-off happened only in America?

Estimates of the precolumbian population of American range from 8 to 145 million. And as one demographer put it, all of them are “completely unscientific.” We just don’t know.

The Willamette Valley in Oregon was routinely burned, too. That’s why it was mostly cleared by the time the first white settlers arrived to farm. In fact, that’s one reason why it looked like such fantastic farm land to them (and, they were right).

Ah. They know the local Indian population was wiped out there because they didn’t find any evidence of them — the wiping-out had been too efficient.

Regarding the whole sub-thread about AmerIndian mortality, I tried Googling “Huron mortality agriculture,” since no-one had mentioned the Huron/Iroquoian nations–both of which groups are well-known as agricultural. (Slash & burn cultivation of corn.) Evidence is indeed sparse, though I did find some support for the 85-90% mortality figure. But not much help with the central question we have in hand, which is, what was the climatic impact of pre-industrial humanity? (BTW, the desertification of the Sahara has occurred since Roman times–what is known about this process, esp, human contributions?)

However, another point about the AmerIndian experience arose which is perhaps relevant to our larger concern–the effects of AGW on humanity’s future. This point is that the effects of European contact were often synergistic. In the case of the Huron/Iroquioan conflict, for example, it is believed that the effects of disease, the increased lethality of weaponry improved by European technology, and intertribal warfare (also entangled with European colonial rivalries) were mutually reinforcing. And in a more specific case I found discussed, a group of Brazilian Kayapo were wiped out over period of a few decades; one significant episode was a measles epidemic which killed 34% of *vaccinated* individuals, including all but two adults over 40. This latter was decisive, because cultural transmission was predominantly grandparent to grandchild; thus social cohesion was severely undermined, and the ground was prepared for much indirect mortality via conflict, dispersal, and loss of technical information (in the sense of agricultural, hunting, food-handling, and other cultural techniques.)

It would certainly appear that *our* social cohesion will be at severe risk with the various predicted physical effects of AGW. “If the sea rise doesn’t get you, the food riots will.” (Not trying to be “alarmist,” just an illustrative comment!) Not for nothing has the U.S. Defense Department begun to evaluate AGW as a security threat. And of course, conflict will make it much harder to deal with the problem: the conflict in Iraq, for one obvious example, has sucked up massive amounts of resources and nearly monopolized years of attention, as well as complicating efforts at cooperation in many other areas. Future conflicts will certainly have parallel effects, presumably at times when we can afford it still less than at present.

this is slightly OT, but could someone of you (maybe Stefan) have a look at this scholarly looking (german) website: http://tinyurl.com/67bel3 . The guy gives in his Holocene time table some rather high values for temperatures for “Roman Optimum” (“Roemisches Optimum”), MWP and LIA. Numbers are 4K, 1 to 1.5 K and -2K resp., for the periods, globaly on average. Rather high I’d say. He presents a lot of informations and it’s unclear to me if he is confused about the numbers, or it’s denialist’s stuff. Anyway, his website is cited by denialist’s so maybe someone could have a look and maybe comment on it.

> Why is it that this kind of [contiental] die-off happened only in America?

No other continent had been geographically isolated for that long and had the population without immunity and the population density for efficient transmission of smallpox when it was introduced. You need all three.

But note the epidemics introduced into many geographically isolated areas, in many species.

A good recent starting point would be this article, its footnotes, and citing papers. It reminds me of Gavin’s point after returning from China a while back about the need for more cross-correlation among people working on paleo core drills. Look at the earlier literature on the Sahara and you’ll find this is quite an update, I think, on what used to be thought.

Excerpt below is from the accompanying Podcast interview — see main link above.:

Interviewer – Robert Frederick
So what did your team look at in order to describe how the Sahara region evolved over
the past 6,000 years?

Interviewee – Stefan Kröpelin
There’s two ways. I mean, one approach we did – it was in a Science paper two years ago where we went into the big surfaces – that means we checked an area of, of much more than 1 million square kilometers for prehistoric sites. And we were locating them and dating them, and this gave us a quite nice picture on the evolution of the eastern Sahara, over the time. But there is practically no more information about the past 3,000 years because even all the humans had left because there was no more rain. So, the only other way is to look into geological deposits. And the most important climate and environment archives are lake deposits. And the good thing about the deposits of Lake Yoa in Ounianga in the extreme northeastern part of the Republic of Chad is that there are not only deposits deposited, accumulated to the present day – that means while we are speaking there are still some thin deposits of the present day sediments and pollen and other indicators. But, the most important thing is that they are vast – that means they are annually or even subannually eliminated, which gives very, very detailed information about environmental and climatic change up to the present day.

Interviewer – Robert Frederick
Do these indicators at all contradict one another as to the condition at this Lake Yoa?

Interviewee – Stefan Kröpelin
Yes, the data, in direct opposition to interpretations, taken from the ocean cores of the western Saharan coast. And these data were interpreted as an arid abrupt change to very arid conditions, which were supposed to have concerned all of the Saharan desert. And this was always not very convincing to people who work in the Sahara because the people from the marine community, of course, they don’t know the terrestrial situation on the continent. So, there was always a suspicion that they might be wrong in their interpretation because they were only using data which they found in marine cores, which so far could not be compared to any data from the continents. And now we are lucky to have this permanent core at a extremely high temporal resolution, well the best you could wish for as a paleoclimatologist. And these data are in clear contrast to the destruction or to the claim of the hypothesis of an abrupt change of the humid period, which was called by some authors African Humid Period. And a change to arid conditions as they are today. It was a rather gradual change, which took some time, and which was very slowly, that means the Sahara turned green around 12,000 years ago. But then, after it had passed the peak of humidity, then it was very slowly becoming a desert today – and this is a gradual process which keeps on to the present day….

Re Barton @ 85: “Why is it that this kind of die-off happened only in America?”

Because the Amerindian population was never exposed to the domesticated animal vectors that harboured the diseases they were exposed to when the Europeans arrived, and thus had developed no natural immunity through natural selection and adaptation. No oxen, no cattle, no horses, no sheep, no swine, no chickens, and no Asian rats and their fleas, and thus not the pathogens that those animals hosted.

Gavin’s response appears to reference two of Steve McIntyre’s posts at his ClimateAudit.org blog, not here at RC. I actually participated in that thread and tried to patiently explain that Steve had almost certainly gotten it wrong, and owed Gavin an apology (a couple of others expressed the same opinion).

It all reminds me of what my high school math teacher would say when confronted with lack of comprehension of the obvious: “Eyes that see not.”

I’m now moderately certain that precession, controlling the thermal equator, is the major influence on the latitude of the ITCZ. (Obviously there are other influences, since in many areas the ITCZ isn’t on the thrmal equator.)

Now for the long-range predictions: since precession is moving the thermal equator to the north, and has been for about 2000 years, the southern portions of the Sahara will revert to (largely) savanna once again; so will the Amazon Basin. I’m much more confident of the latter; Dr. Jose Mendoza writes so and it seems there was a paper at last winter’s AGU meeting along these lines; the last time the precessional cycle brought the thermal equator to the north the Amazon Basin was mostly savannah and also the time before that.

Now there is nothing to do but adapt to this. However, it probably will take about 5-6 thousand years, so there is plenty of time. That said, possibly anybody attempting to do a regional climate forecast for 2200 CE might do well to take this precessional effect into account.

Apart from the question of who needs to apologize, I’m not sure that anyone is disagreeing about the facts, just the apportionment of blame. Is this your understanding of the time line?

1. An original version of the data was posted that included some degree of infilling. Steve McIntyre downloaded the data, and griped publicly about the infilling.
2. At some point after this (04-Sep-2008 15:14, likely UTC, but definitely after McIntyre’s initial download), Gavin provided a link to the newly availableraw data at the SI site.
3. That night, McIntyre posts a vituperative comment asserting that the data is available only in an infilled form. At the time of the comment, this is false, as the raw data has become available since his download.
4. The next morning, McIntyre posts a slightly more polite comment #45 here repeating his outdated accusations, to which Gavin responds inline suggesting a timeline of events consistent with this one.

My read is that McIntyre was caught unaware when the data was changed at some point day, and Gavin is offended by the accusations that anything he posted was false at the time it was posted. Does this agree with your interpretation of events?

#97 Nathan
I agree that the timelines are pretty much as you and Gavin described them, subject to the comments below (edited from previous posts at ClimateAudit).

It well may be that the raw proxies only appeared after objections from Steve McIntyre and others. But, of course, that still does not support the specific allegation that was made against Gavin Schmidt by Steve McIntyre, and nor does it excuse the vituperative attacks that are all too common at CA.

Now it’s clear that there was some sort of problem with the first Sept. 4 “originalallproxy” archive, and so it was updated on Sept. 5. My personal opinion is that these kinds of issues should be documented in a directory “readme” and resolved if necessary by respectful, professional dialogue between investigators.

The continual unwarranted presumption at CA of bad faith, expressed publicly in unprofessional and abusive language, is a severe impediment to meaningful dialogue about these kinds of issues, or indeed any scientific issues.